1. Field of the Invention
The present invention relates to a method of making a highly defined bilayer lift-off mask and, more particularly, to a method wherein lift-off mask material is subjected to an electron beam for decreasing the molecular weight of a bottom release layer and increasing the molecular weight of a top photoresist layer so that a weak developer can be employed for patterning the bottom release layer and ion milling does not alter a track width of the top photoresist layer.
2. Description of the Related Art
Magnetic head assemblies are typically made of multiple thin film layers which are patterned to form various shaped layers in the head. Some of the layers are plated while other layers are sputter deposited on a wafer substrate. The read head portion of a magnetic head assembly includes multiple layers that are typically sputter deposited. For example, the multiple layers of a read sensor, hard bias and lead layers connected to the sensor and first and second read gap layers below and on top of the sensor are typically sputter deposited. A prior art method of forming shaped sputter deposited layers is to sputter deposit a full film layer of the required material on a wafer substrate, form a patterned photoresist layer on the layer, ion mill away the exposed portion of the layer and then remove the photoresist layer leaving the desired shaped layer that was protected therebelow.
The aforementioned method of shaping sputter deposited layers has been generally superseded by a bilayer lift-off mask scheme which is fully explained in commonly assigned U.S. Pat. No. 5,018,037 which is incorporated by reference herein. The bilayer lift-off mask has a T-shape as seen in cross section wherein the vertical portion of the T is short and wide but less wide than the horizontal top portion of the T. The top portion of the T is generally a patterned photoresist layer and the bottom vertical portion of the T is a release layer. The configuration provides first and second undercuts as seen in cross section wherein each undercut has a height and a length below the top photoresist portion. In the aforementioned patent the bilayer lift-off mask is employed for the purpose of making contiguous junctions of the first and second lead layers with first and second side edges respectively of the read sensor. Multiple read sensor layers are sputter deposited in full film on the wafer substrate followed by formation of the bilayer lift-off mask covering a read sensor site. Ion milling is then employed to remove all of the read sensor material except that below the mask. Full films of hard bias and lead layer materials are then sputter deposited which cover the top of the lift-off mask and an area surrounding the lift-off mask. It is important that the height of the undercuts be greater than the thickness of the hard bias and lead layers. This is so a photoresist stripper can reach the bottom release layer. The stripper is then introduced which dissolves the bottom release layer causing the bilayer lift-off mask and the hard bias and lead materials deposited thereon to be released from the wafer substrate leaving the aforementioned contiguous junctions between the first and second lead layers and the first and second side edges respectively of the read sensor.
The aforementioned method prior to the aforementioned patent was not precise enough to implement contiguous junctions between the read sensor and the lead layers. Prior to the patent the lead layers overlapped the top of the read sensor and were constructed with a second photoresist mask. Since photopatterning of photoresist masks is not precise enough to align a second mask with side walls created by a first mask, the overlapping scheme was necessary. Unfortunately, this scheme caused the hard bias and lead layers to form a high profile on top of the read sensor which was replicated through subsequent layers into a write gap of the write head causing a curvature of the write gap. Write gap curvature degrades the performance of the head since the write head writes curved magnetic bits of information into the rotating disk while the read head reads the magnetic bits of information straight across. This causes a loss of signal at the outside lateral edges of the track width of the read head.
Accordingly, the bilayer lift-off mask scheme has significantly improved the making of read heads by forming contiguous junctions between the lead layers and the read sensor. Less processing steps are required and the profile of the lead and hard bias layers above the read sensor has been reduced. Unfortunately, present bilayer lift-off masks are limited to construction of read heads with a track width of greater than one micron. The more narrow the track width the greater the tracks per inch (TPI) that can be read by the read head from a rotating magnetic disk. Accordingly, the greater the tracks per inch the greater the storage capacity of a disk drive employing such a read head. Processing control of the length and height of the undercut has not been precise enough for submicron track widths. Long first and second undercuts leave insufficient release layer material which can cause the bilayer lift-off mask to be separated from the substrate or topple over during subsequent processing steps of ion milling and sputter deposition. If the undercut is too short fencing can occur which is deposition of the sputtered material across the height of the undercut preventing the photoresist stripper from reaching the underlying release layer. These problems have generally been caused by strong photoresist developers employed to pattern the release layer. Because of the rapid removal of the light exposed release layer portion by strong photoresist developers it has been difficult to precisely stop the removal of the release layer portions forming the aforementioned undercuts.
Another problem with the strong photoresist developer is that in bilayer photopatterning steps subsequent to formation of the leads the photoresist developer will attack leads made of aluminum copper (AlCu). Still another problem has been the height control of the undercuts. Still a further problem is that the strong photoresist developer attacks the top photoresist layer reducing its width. This reduction in width equates to a reduction in the track width of the read head. Typical processing of prior art bilayer lift-off masks has been to treat a single layer of photoresist with ultraviolet or an electron beam to a particular depth. Unfortunately, the penetration depth of the beam has not been precise enough to form a highly defined bottom release layer portion of a single resist layer. The height control is important since the height must be greater than the thickness of the sputter deposited material so that a photoresist stripper can reach the bottom release layer for lift-off purposes. Still a further problem with the present processing of bilayer lift-off masks is that the ion milling step reduces the width of the top photoresist layer portion. This is serious problem because this reduction reduces the track width of the read head in an uncontrolled manner.
Accordingly, there is a strong-felt need for a process of making a bilayer lift-off mask which has undercuts with precise heights and lengths, better control over a track width of the top photoresist layer portion during ion milling and provision of a photoresist developer which will not attack leads made of aluminum copper (AlCu).